Translated translational operator in Escherichia coli. Auto-regulation in the infC-rpmI-rplT operon.

The genes coding for translation initiation factor IF3 (infC) and for the ribosomal proteins L35 (rpmI) and L20 (rplT) are transcribed in that order from a promoter in front of infC. The last two cistrons of the operon (rpmI and rplT) can be transcribed from a weak secondary promoter situated within the first cistron (infC). Previous experiments have shown that the expression of infC, the first cistron of the operon, is negatively autoregulated at the translational level and that the abnormal AUU initiation codon of infC is responsible for the control. We show that the expression of the last cistron (rplT) is also autoregulated at the posttranscriptional level. The L20 concentration regulates the level of rplT expression by acting in trans at a site located within the first cistron (infC) and thus different from that at which IF3 is known to act. This regulatory site, several hundred nucleotides upstream from the target gene (rplT), was identified through deletions, insertions and a point mutation. Thus, the expression of the operon is controlled in trans by the products of two different cistrons acting at two different sites. The localization within an open reading frame (infC) of a regulatory site acting in cis on the translation of a downstream gene (rplT) is new and was unforeseen since ribosomes translating through the regulatory site might be expected to impair either the binding of L20 or the mRNA secondary structure change caused by the binding. The possible competition between translation of the regions acting in cis and the regulation of the expression of the target gene is discussed.